That's because $ENV is decided by the enviornment the script is running in. See antirice's post above this. Note that $ENV is different depending upon your server OS, and whatever else happens to be floating around on the system. Also note that there is a lot more stuff in $ENV when running a script on a webserver, then just running it on the command line. Printing all the keys in the enviornment you will use is basically the only way to be sure you know what's available.